1. Field of the Invention
This invention relates to containers and container ends, and in particular to a tabless opening device in a container wall and a method and tools for forming the same.
2. Description of the Prior Art
The problem of litter caused by completely severable tear strips and pull tabs on easy opening cans has resulted in the development of several tabless easy opening devices in which no element is completely severable from the container wall. Disclosures of such tabless opening devices are contained in U.S. Pats. Nos. 3,227,304, 3,246,791, 3,355,058, 3,334,775, 3,362,569 and 3,410,436 among others.
It is known to form a weakening line for such a tabless opening device by displacing metal along one side of a weakening line at right angles to the initially undisplaced surface of a container wall in such a way that the edge defining the periphery of a removable or partially removable wall portion underlaps the corresponding edge of the non-removable wall portion and has an abrupt change in cross-sectional wall thickness as is disclosed in U.S. Pat. No. 3,362,569. It is further known to form a weakening line around a partially removable panel in a container wall by moving metal substantially at right angles to the original surface of the sheet and squeezing a narrow zone of metal along such line to cause lateral flow of material away from the line as is disclosed and claimed in an application for U.S. Letters Patent Ser. No. 357,937, filed May 7, 1973. Although the method and apparatus disclosed in such patent application work well for material having any given thickness, the dies used in such method have sometimes been found to produce defective ends when the sheet material from which they are formed varies in mechanical properties and/or in thickness within commercially accepted ranges. Sheet metal from which container ends are formed may have tolerances of approximately plus or minus 0.0005 inch in aluminum and even more in steel, and such tolerances can cause difficulties in forming weakening lines in the metal using the prior art dies. When such sheet metal is thicker than nominal gauge, the ends produced with prior art dies have sometimes been difficult to open, and when the material is thinner than nominal, the tabless container ends produced from such material have sometimes had cracks or premature failures in the weakening lines.
The prior art is lacking in a method and tools for making tabless easy opening devices which work well on deformable sheet material which varies in thickness within commercially accepted ranges.